President Emmanuel Macron cancelled a major televised policy speech he was due to give on Monday evening over the "terrible fire ravaging Notre-Dame."
He headed to the scene in person.
In a tweet he expressed the "emotion of a whole nation" on seeing Notre-Dame ablaze.
"Like all my compatriots I am sad to see a part of us burn this evening" he said, expressing solidarity with "all Catholics and all French people."
"Notre Dame belongs to the whole of humanity. It has inspired writers, painters. philosophers and visitors who have come from all round the world," said EU Commission head Jean Claude Juncker
EU Council president Donald Tusk added: "Notre Dame of Paris is Notre Dame of the whole of Europe, We are all with Paris today."
British Prime Minister Theresa May expressed her thoughts for the people of France and emergency services battling a devastating fire that has engulfed the Notre-Dame Cathedral in Paris on Monday.
"My thoughts are with the people of France tonight and with the emergency services who are fighting the terrible blaze at Notre-Dame Cathedral," the British leader tweeted.
My thoughts are with the people of France tonight and with the emergency services who are fighting the terrible blaze at Notre-Dame cathedral.
— Theresa May (@theresa_may) April 15, 2019
German Chancellor Angela Merkel called Notre-Dame cathedral a "symbol of European culture" as the blaze raged.
There was no immediate indication of any casualties in the blaze.
"The Paris fire service is trying to control the flames," Paris's Mayor Anne Hidalgo wrote on Twitter, asking residents to respect the security cordon thrown up around the site.
A spokesperson for the cathedral said the fire, which comes as French Catholics prepare to celebrate Easter, broke out at around 1650 GMT.
US President Donald Trump in a tweet said: "So horrible to watch the massive fire at Notre Dame Cathedral in Paris. Perhaps flying water tankers could be used to put it out. Must act quickly!"
So horrible to watch the massive fire at Notre Dame Cathedral in Paris. Perhaps flying water tankers could be used to put it out. Must act quickly!
— Donald J. Trump (@realDonaldTrump) April 15, 2019